Since display devices using light-emitting elements have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angles, they have attracted attention as display devices that supersede cathode ray tubes (CRTs) and liquid crystal display devices. Specifically proposed structures of active matrix display devices using light-emitting elements are different depending on manufacturers. In general, a pixel includes at least a light-emitting element, a transistor that controls input of video signals to the pixel (a switching transistor), and a transistor that controls the amount of current supplied to the light-emitting element (a driving transistor).
When all the transistors in pixels have the same polarity, it is possible to omit some of steps for fabricating the transistors, for example, a step of adding an impurity element imparting one conductivity type to a semiconductor film. Patent Document 1 discloses a light-emitting element type display in which transistors included in pixels are all n-channel transistors.